Modern electronic equipment relies heavily on printed circuit boards on which semiconductor chips, or integrated circuits (ICs), are mounted. The mechanical and electrical connections between the chip and the substrate have posed challenges for chip designers. Three well known techniques for interconnecting the IC to the substrate are: wire bonding, tape automated bonding (TAB) and flip-chip.
The most common of these processes is wire bonding. In wire bonding, a plurality of bonding pads are located in a pattern on the top surface of the substrate, with the chip mounted in the center of the pattern of bonding pads, and the top surface of the chip facing away from the top surface of the substrate. Fine wires (which may be aluminum copper or gold wires) are connected between the contacts on the top surface of the chip and the contacts on the top surface of the substrate. Particularly, the connecting wires are supplied and bonded to the chip and to the substrate through a capillary, a bonding tool further described below.
Capillaries (bonding tools) are used for ball bonding the wire to electronic devices, particularly to bond pads of semiconductor devices. Such capillaries are generally formed from a ceramic material, principally aluminum oxide, tungsten carbide, titanium carbide, cermets, ruby, zircon toughened alumina (ZTA), alumina toughened zircon (ATZ). Very thin wire, generally on the order of about one mil gold, copper or aluminum wire, is threaded through an axial passage in the capillary with a small ball being formed at the end of the wire, the ball being disposed external of the capillary tip. The initial object is to bond the ball to a pad on the semiconductor device and then to bond a portion farther along the wire to a lead frame or the like. During the bonding cycle, the capillaries perform more than one function.
After the ball is formed, the capillary must first center the ball partly within the capillary for bond pad targeting. With a first bonding step, the ball is bonded to a pad on a semiconductor device. When the capillary touches the ball down on the bond pad, the ball will be squashed and flatten out. As the bond pads are generally made from aluminum, a thin oxide forms on the surface of the bond pad. In order to form a proper bond, it is preferable to break the oxide surface and expose the aluminum surface. An effective way of breaking the oxide is to “scrub” the surface of the oxide with the wire ball. The wire ball is placed on the surface of the aluminum oxide and the capillary rapidly moves in a linear direction based on the expansion and contraction of a piezo-electric element placed within the ultrasonic horn to which the capillary is attached. The rapid motion, in addition to heat applied through the bond pad, forms an effective bond by transferring molecules between the wire and the bond pad.
The capillary then handles the wire during looping, smoothly feeding the bond wire both out of the capillary and then back into the capillary. The capillary then forms a “stitch” bond and a “tack” or “tail” bond.
Presently, thermosonic wire bonding is the process of choice for the interconnection of semiconductor devices to their supporting substrates. The thermosonic bonding process is partially dependent upon the transfer of ultrasonic energy from the transducer, attached to a movable bondhead, through a tool, e.g. capillary or wedge, to the ball or wire being welded to the semiconductor device or supporting substrate.
FIG. 1 is an illustration of a well-known prior art fine pitch bonding tool 100. As shown in FIG. 1, bonding tool 100 is formed from a unitary piece of material having a cylindrical portion 101, a tapered portion 102 coupled between cylindrical portion 101, and working tip 104. To meet the requirement of electrical isolation between the bonding machine and the device being fabricated, the material used to form bonding tool 100 is non-conductive. Typically, this non-conductive material is a brittle ceramic based compound such as alumina, for example.
U.S. Pat. Nos. 5,871,141, 5,558,270, and 5,421,503 assigned to the same assignee as the present invention, describe various conventional bonding tools for producing wire bonds on semiconductor devices and are incorporated herein by reference.
Conventional bonding tools are formed from non-conductive materials, such as alumina, and include a working tip used to form the bonds between the contact pad and the bonding wire. Such non-conductive bonding tools are necessary in order to protect the substrate being bonded from potential electrical discharges from the bonding machine. Conventional bonding tools are deficient, however, because the ceramic materials from which they are formed are brittle and do not lend themselves to be fabricated with working tip features that allow them to form bonds in ultra fine pitch applications so as to provide a high level of inter metallic coverage between the bonding ball formed by the tool and the bonding pad. Furthermore, conventional bonding tools are formed as a unitary part. It use, however, only the working tip of the capillary becomes worn, but requires the replacement of the entire part, thereby wasting valuable material. Further, it is more difficult to manufacture such a unitary bonding tool. For ease of manufacturing and/or replacement purposes, it would advantageous to provide a bonding tool where the working tip is separate from the shaft.
The inventors of the present invention have developed a bonding tool that meets the demands imposed by these high-density devices while maintaining structural integrity of the bonding tool.